A magnetic memory (called an MRAM) including a conventional magnetoresistance effect element is described.
FIG. 24 shows a memory cell of the magnetic memory including the conventional magnetoresistance effect element. As shown in FIG. 24, a magnetic memory cell 2100 has a structure where a magnetoresistance effect element 2101 and a selection transistor 2102 are electrically connected in series. The selection transistor 2102 has a source electrode electrically connected to a source line 2103, a drain electrode electrically connected through the magnetoresistance effect element 2101 to a bit line 2104, and a gate electrode electrically connected to a word line 2105.
The magnetoresistance effect element 2101 basically has a three-layer structure with a first ferromagnetic layer 2106, a second ferromagnetic layer 2109, and a first non-magnetic layer 2110 provided between the first and second ferromagnetic layers 2106 and 2109. The resistance value of the magnetoresistance effect element is reduced if the respective magnetizations of the first and second ferromagnetic layers 2106 and 2109 are parallel and is increased if these magnetizations are antiparallel.
The memory cell 2100 of the magnetic memory assigns these two resistance conditions to bit information “0” and bit information “1.” The magnetization direction of the first ferromagnetic layer 2106 is fixed. The magnetization of the second ferromagnetic layer 2109 is reversed by a spin transfer torque generated by a current supplied from the selection transistor. The current flowing from bottom to top of FIG. 24 results in antiparallel arrangement of the magnetizations. The current flowing in the opposite direction results in parallel arrangement of the magnetizations. Thus, the respective magnetization directions of the first and second ferromagnetic layers 2106 and 2109 can be changed to parallel arrangement or antiparallel arrangement in response to the direction of the current supplied from the selection transistor 2102. This changes the resistance value of the magnetoresistance effect element 2101, thereby allowing writing of bit information.
In the aforementioned example, the first ferromagnetic layer 2106 is described as a reference layer (also called a fixed layer) of fixed magnetization and the second ferromagnetic layer 2109 is described as a recording layer (also called a free layer) of a variable magnetization direction. Even if this structure is reversed, the function of the MRAM is still maintained.
In order to achieve the MRAM, the following three characteristics should be satisfied at the same time: the magnetoresistance effect element 2101 as a recording element should have a magnetoresistance change ratio (MR ratio) of 100% or more; the magnetization of the second ferromagnetic layer 2109 to become the recording layer should be reversed with a writing current lower than a drive current of the selection transistor (assuming that the gate width of the selection transistor is F nm, the drive current is about F μA); and the second ferromagnetic layer 2109 to become the recording layer should have a thermal stability constant (E/kBT, where E is an energy barrier, kB is Boltzmann's constant, and T is the absolute temperature (K)) of 70 or more.
In a film structure of the magnetoresistance effect element 2101 known to achieve a high magnetoresistance change ratio, the first and second ferromagnetic layers 2106 and 2109 are formed of a material having the bcc structure including any one of 3d transition metal elements such as Fe, Co and Ni, and MgO is used for the first non-magnetic layer 2110. If the magnetoresistance effect element is made by using these materials, the first and second ferromagnetic layers 2106 and 2109 generally become thin compared to the size of the element. This makes a large demagnetizing field act in a direction perpendicular to a film surface. As a result, the respective magnetizations of the first and second ferromagnetic layers 2106 and 2109 point in a direction parallel to the film surface.
In order to reverse the direction of a magnetization using a spin transfer torque while the magnetization points in the direction parallel to the film surface, an energy barrier caused by the large demagnetizing field existing in the direction perpendicular to the film surface should be overcome. This leads to the problem of increase in a switching current.
This problem may be solved by the means of pointing the respective magnetization directions of the first and second ferromagnetic layers 2106 and 2109 in the perpendicular direction. In doing so, during magnetization reversal using a spin transfer torque, the demagnetizing field reduces the energy barrier caused during the magnetization reversal by the spin transfer torque. This allows reduction in the switching current.
FIG. 25 is a sectional view showing the structure of a magnetoresistance effect element 2101 reported in non-patent literature 1. As shown in FIG. 25, the magnetoresistance effect element 2101 includes a foundation layer 2201, a reference layer formed of a first ferromagnetic layer 2106 on the foundation layer 2201, a first non-magnetic layer 2110 formed on the reference layer, a recording layer formed of a second ferromagnetic layer 2109 on the first non-magnetic layer 2110, and a cap layer 2202 on the recording layer.
In the structure of the magnetoresistance effect element 2101 of non-patent literature 1, CoFeB is used for the first and second ferromagnetic layers 2106 and 2109, MgO is used for the first non-magnetic layer 2110, and a film of CoFeB is thinned. By using magnetic anisotropy generated at an interface with the MgO layer in the first non-magnetic layer 2110, magnetization can point in a direction perpendicular to a film surface.
Non-patent literature 1 reports that the aforementioned structure can achieve a thermal stability index of 40 and a low writing current of 49 μA in a device of a diameter of 40 nm while a high magnetoresistance change ratio (also called an MR ratio) of 100% or more is maintained. The thermal stability index of 40 is a numerical value sufficient for retaining one-bit information for 10 years. However, this value keeps a problem unsolved in that it is lower than a thermal stability index of 70 required to achieve an MRAM.
FIG. 26 is a sectional view showing the structure of a magnetoresistance effect element 2300 disclosed in patent literature 1. As shown in FIG. 26, the magnetoresistance effect element 2300 disclosed in FIG. 9 of patent literature 1 includes a foundation layer 2503, a fixed layer 2310 on the foundation layer 2503, a non-magnetic layer 2370 on the fixed layer 2310, a recording layer 2380 formed of a ferromagnetic layer on the non-magnetic layer 2370, and a cap layer 2504 on the recording layer 2380. The fixed layer 2310 has a five-layer structure with a non-magnetic layer 2311, a ferromagnetic layer 2312, a non-magnetic layer 2313, and a ferromagnetic layer 2314. Patent literature 1 discloses that making the magnetization direction of the fixed layer perpendicular to a film surface achieves stability, specifically enhances thermal stability. Patent literature 1 describes that the non-magnetic layer 2313 in the fixed layer 2310 generates magnetic coupling between the ferromagnetic layer 2312 and the ferromagnetic layer 2314, and that specific materials for the non-magnetic layer 2313 include an insulator such as MgO, Al2O3 or SiO2, and metal made of Ru, Rh, V, Ir, Os or Re.